The present invention generally relates to a dielectric filter of a solid mold type to be used at frequencies of, for example, several hundred MHz, and more particularly, to a filter which has a frequency adjustment structure thereon which is able to conform the resonance frequencies of the respective resonators with better accuracy.
Conventionally, it is known to use a dielectric filter of a solid mold type as a filter at frequencies of several hundred MHz. Such a dielectric filter has a plurality of through holes formed in parallel in the dielectric block, has inner conductive films formed on the inner peripheral faces of the through holes, and has outer conductive films formed respectively on the outer side faces of the block and associated with the through holes so as to constitute at least a pair of resonators.
It is important for the respective resonators of this solid mold type of dielectric filter to be conformed with better accuracy to each other, as regards their resonance frequencies, corresponding to the desired filter characteristics. However, if there are three or more stages the frequency of a middle resonator is likely to become higher as compared with the resonators on the input- and output-side ends.
Thus, a frequency-regulation construction has been proposed which causes the frequency of the respective resonators to be conformed to each other. For example, as shown in FIG. 7, according to one method of forming electrodes 33a, 33b for performing such an adjustment, inner conductive films 32 extend onto the end face 31a on the open side of the dielectric block 31, with the inner conductive films also being formed on the inner peripheral faces of the through holes 31b. In this method, the electrodes 33a, 33b are usable for this adjustment, since they are formed in such shape and size as will provide the desired resonance frequencies.
However, the above-described conventional frequency adjustment construction has the following problems.
(1) The desired sizes of the adjustment electrodes are hard to produce accurately, thus resulting in lower adjustment accuracy of the frequency, in that the face on the open side of the dielectric block, on which each adjustment electrode is formed, is generally rough on the surface, whereby the inner conductive film extending from the inner peripheral face of the through hole may rise upwardly from the top face as a result of such rough surface, when a convenient manufacturing method is used.
(2) As the adjustment electrode is adapted to be connected with the inner conductive film at the top edge portion of the through hole, the connection may become incomplete because of a break, crack, or the like, of the edge portion, with the result that an undesirable frequency dispersion and/or Q deterioration are likely to be caused.